Digital versatile discs (DVDs) are known as optical discs capable of storing large-capacity data, since they can store digital information with recording densities of about 6 times those of compact discs (CDs). In recent years, along with increases of amounts of information to be recorded in optical discs, there has been a need for optical discs with larger capacities. In order to increase the capacity of an optical disc, it is necessary to reduce the size of a light spot formed by light directed to the optical disc during recording information onto the optical disc and during playing back information recorded on the optical disc to increase the information recording density. The size of the light spot can be reduced, by employing laser light with a short wavelength from a light source and also employing an objective lens with a larger numerical aperture (NA). For DVDs, a light source with a wavelength of 660 nm and an objective lens with a numerical aperture (NA) of 0.6 are used. Further, by employing a blue laser with a wavelength of 405 nm and an objective lens with an NA of 0.85, for example, it is possible to attain a recording density of 5 times the recording density of a current DVD. Further, hereinafter, such an optical disc will be referred to as a BD.
On the other hand, a coma aberration is induced by an inclination of the objective lens with respect to an optical disc, and the degree of the coma aberration increases in proportion to the cube of the numerical aperture. Accordingly, it is necessary to position, with high accuracy, the optical axis of the objective lens with respect to the optical disc, in order to obtain preferable recording/playback signals.
Therefore, there have been contrived various methods for driving an objective lens to tilt it for inclining the objective lens for correcting coma aberrations, in an objective-lens actuator incorporating the objective lens for driving the objective lens in a focusing direction and in a tracking direction. Further, conventionally, coma-aberration correction by tilt-driving of an objective lens is for correcting coma aberrations in a radial direction (the direction of a radius) of an optical disc.
Further, there have been suggested various methods for correcting coma aberrations by placing an aberration-correction device on an optical path between a light source and an objective lens, rather than by inclining an objective lens for correcting coma aberrations.
For example, a liquid crystal device having a coma-aberration correcting function can be placed on an optical path between a light source and an objective lens, in order to adjust the amount of coma-aberration correction through the voltage applied to the liquid crystal device.
However, the coma-aberration correcting method using the liquid crystal device can correct only angular deviations at low frequencies and, therefore, has the problem of difficulty in reducing coma aberrations induced over a wide range from a low frequency to a high frequency. Further, the coma-aberration correcting method has the problem that a liquid crystal device is more expensive than a lens, which prevents cost reduction.
Further, there has been a device as follows, as a conventional technique which discloses another method for correcting coma aberrations by placing an aberration correcting device on an optical path from a light source to an objective lens (refer to Patent Citation 1).
According to the Patent Citation 1, a coma-aberration correction actuator incorporating an aberration-correction lens as an aberration correction device is placed on an optical path from a light source to an objective lens, and the amount of the inclination of the aberration correction lens with respect to the optical axis is controlled for correcting coma aberrations. The coma-aberration correction actuator is constituted by three VCMs (Voice Coil Motors) placed at substantially equal intervals in a circumferential direction of the aberration correction lens. By controlling the respective amounts of movements of the three VCMs, the amount of the tilt (the amount of the inclination) of the aberration correction lens is controlled, which enables reduction of coma aberrations induced over a wide range from a low frequency to a high frequency.
Patent Citation 1: JP-A No. 2002-140831